1. Field of the Invention
This invention relates to a device for storing energy. In one aspect, this invention relates to capacitors and supercapacitors. In another aspect, this invention relates to a method and means for reducing or eliminating the self-discharge of capacitors and supercapacitors.
2. Description of Related Art
A capacitor, also referred to as a condenser, is an electric circuit element used to store charge (energy) temporarily, consisting in general of two metallic plates, i.e. positive and negative electrodes, separated and insulated from each other by a dielectric. In a conventional capacitor, energy is stored by the removal of charge carriers, typically electrons, from one metal plate and depositing them on the other metal plate. This charge separation creates a potential between the two plates, which can be harnessed in an external circuit. The total energy stored in this fashion (capacitance) is a combination of the number of charges stored and the potential between the plates. The former is essentially a function of size and the material properties of the plates, whereas the latter is limited by the dielectric breakdown between the plates. Various materials can be inserted between the plates to enable higher voltages to be stored, leading to higher energy densities for any given size.
Supercapacitors, also known as electric double-layer capacitors (DLC), electrochemical double layer capacitors (EDLC), or ultracapacitors, are electrochemical capacitors that have an unusually high energy density when compared with conventional capacitors, typically on the order of thousands of times greater than a high-capacity electrolytic capacitor. In contrast with conventional capacitors, supercapacitors do not have a conventional dielectric, as such. Rather, they are based on a structure that contains an electrical double layer. FIG. 1 shows a comparison of a conventional capacitor and a supercapacitor.
Energy storage in a capacitor is by means of a static charge rather than by an electrochemical process that is inherent to batteries. Applying a voltage differential on the positive and negative electrodes charges the capacitor or supercapacitor. However, whereas a conventional capacitor consists of conductive foils and a dry separator, supercapacitors employ special electrodes and some electrolyte.
In an electric double-layer capacitor, the effective thickness of the “dielectric” is exceedingly thin—on the order of nanometers—and that, combined with the very large surface area, is responsible for their extraordinarily high capacitances in practical sizes. In an electrical double layer, each layer by itself is quite conductive, but the physics at the interface where the layers are effectively in contact means that no significant current can flow between the layers. However, the double layer can withstand only a low voltage, which means that supercapacitors rated for higher voltages must be made of matched series-connected individual supercapacitors.
In general, supercapacitors improve storage density through the use of a nanoporous material, typically activated carbon, in place of the conventional insulating barrier. Activated carbon is a powder made up of extremely small and very “rough” particles, which, in bulk, form a low-density volume of particles with holes between them that resembles a sponge. The overall surface area of even a thin layer of such a material is many times greater than a traditional material like aluminum, allowing many more electrons to be stored in any given volume. The disadvantage of using activated carbon is that the carbon is taking the place of the improved insulators used in conventional devices, as a result of which supercapacitors employ low potentials on the order of only 2 to 3 V.
There are three types of materials suitable for use as electrodes in a supercapacitor—high surface area activated carbons, metal oxides, and conducting polymers. The high surface area electrode material is the most common and least costly to manufacture. In a double layer capacitor, energy is stored in a double layer formed near the carbon electrode surface. The electrolyte employed in a double layer capacitor may be aqueous or organic. Using an aqueous electrolyte provides low internal resistance but limits the voltage to about 1 volt. In contrast thereto, using an organic electrolyte provides about 2.5 volts of charge but the internal resistance is high. Thus, to operate at higher voltages, supercapacitors may be connected in series. The gravimetric energy density of the supercapacitor is about 1 to 10 Wh/kg, which is high compared with conventional capacitors but constitutes only about one-tenth of the energy density of a nickel-metal-hydride battery. Whereas the electrochemical battery delivers a fairly steady voltage in the usable energy spectrum, the voltage of the supercapacitor is linear and drops evenly from full voltage to zero volts. As a result, supercapacitors are unable to deliver a full charge. Thus, supercapacitors have high power densities (W/kg) but low energy density (Wh/kg).